In modern cellular radio systems, the radio network has a strict control on the behavior of the terminal. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from the base station to the terminal.
At power-on or after a long standby time, the user equipment (UE) is not synchronized in the uplink. The UE can derive an uplink frequency and power estimate from the downlink (control) signals. However, a timing estimate is difficult to make since the round-trip propagation delay between a base station, eNodeB, and the UE is unknown. So, even if UE uplink timing is synchronized to the downlink, it may arrive too late at the eNodeB receiver because of propagation delays. Therefore, before commencing traffic, the UE has to carry out a Random Access (RA) procedure to the network. After the RA, the eNodeB can estimate the timing misalignment of the UE uplink and send a correction message. During the RA, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a RA procedure.
Usually, a Physical Random Access Channel (PRACH) is provided for the UE to request access to the network. An access burst is used which contains a preamble with a specific sequence with good autocorrelation properties. The PRACH may be orthogonal to the traffic channels. For example, in GSM a special PRACH time slot is defined. Because multiple UEs may request access at the same time, collisions may occur between the requesting UEs. A contention resolution scheme has to be implemented to separate the UE transmissions. The RA scheme usually includes a random back off mechanism. The timing uncertainty is accounted for by extra guard time in the PRACH slot. The power uncertainty is usually less of a problem as the PRACH is orthogonal to the traffic channels.
To distinguish between the different requesting UEs performing RA typically many different RA preambles exist. A UE performing RA picks randomly a preamble out of a pool and transmits it. The preamble represents a random UE ID which is used by an eNodeB when granting the UE access to the network via the eNodeB. The eNodeB receiver can resolve RA attempts performed with different preambles and send a response message to each UE using the corresponding random UE IDs. In case that requesting UEs simultaneously use the same preamble a collision occurs and most likely the RA attempts are not successful since the eNodeB cannot distinguish between the two users.
In E-UTRAN, evolved UMTS Terrestrial Radio Access Network, 64 preambles are provided in each cell. Preambles assigned to adjacent cells are typically different to insure that a RA in one cell does not trigger any RA events in a neighboring cell. Information that must be broadcasted from the base station is therefore the set of preambles that can be used for RA in the current cell.
Since E-UTRAN is capable of operation under very different operation conditions, from femto- and pico-cells up to macro-cells, different requirements are put on RA. Whereas the achievable signal quality for RA is less of a problem in small cells and more challenging in large cells. To also ensure that enough RA preamble energy is received, E-UTRAN defines different preamble formats. Only one such preamble format may be used in a cell and also this parameter must therefore be broadcasted. For Frequency Division Duplex, FDD, four preambles formats are defined.
Yet another parameter that is broadcasted is the exact time-frequency location of an RA resource, also called RA slot or RA opportunity. Such an RA time resource spans always 1.08 MHz in frequency and either 1, 2, or 3 ms in time, depending on the preamble format. For FDD, 16 configurations exist, each defining a different RA time-domain configuration.
In an FDD system, in addition to the signaling required to point out the 64 preambles that can be used in the current cell, another 6 bits are required to indicate preamble format (2 bits) and RA time-domain configuration (4 bits).
Referring to, for example, E-UTRAN time division duplex, TDD, mode, TDD mode has some particularities relative to the FDD mode. These particularities make a simple reuse impossible or impractical including, e.g., that TDD defines in total 5 RA preamble formats and not 4 requiring 3 bits to signal the format.
In FDD the RA time-domain configurations express the first subframe of an RA resource as subframe number within a frame. In an FDD system all subframes located at the UL frequency band are UL subframes at all times and each of them may be—according to the RA time-domain configuration—assigned to RA. In TDD however only a subset of all available subframes are UL subframes and merely those may therefore be allocated to RA. Therefore, the simple counting mechanism based on subframes can not be applied to TDD.